New MEA Materials for Improved DMFC Performance, Durability and Cost

Fletcher, James H.¹  Campbell, Joseph L.¹  Cox, Philip¹  Harrington, William J.¹ 

[1] University of North Florida

关键词: Fuel cell;    Hydrogen;   

DOI  :  10.2172/1092952
RP-ID  :  DOE/EE0000475
PID  :  OSTI ID: 1092952

学科分类：再生能源与代替技术

美国|英语

来源: SciTech Connect

【 摘 要 】

Abstract Project Title: New MEA Materials for Improved DMFC Performance, Durability and Cost The University of North Florida (UNF)--with project partners the University of Florida, Northeastern University, and Johnson Matthey--has recently completed the Department of Energy (DOE) project entitled ???New MEA Materials for Improved DMFC Performance, Durability and Cost???. The primary objective of the project was to advance portable fuel cell MEA technology towards the commercial targets as laid out in the DOE R&D roadmap by developing a passive water recovery MEA (membrane electrode assembly). Developers at the University of North Florida identified water management components as an insurmountable barrier to achieving the required system size and weight necessary to achieve the energy density requirements of small portable power applications. UNF developed an innovative ???passive water recovery??? MEA for direct methanol fuel cells (DMFC) which provides a path to system simplification and optimization. The passive water recovery MEA incorporates a hydrophobic, porous, barrier layer within the cathode electrode, so that capillary pressure forces the water produced at the cathode through holes in the membrane and back to the anode. By directly transferring the water from the cathode to the anode, the balance of plant is very much simplified and the need for heavy, bulky water recovery components is eliminated. At the heart of the passive water recovery MEA is the UNF DM-1 membrane that utilizes a hydrocarbon structure to optimize performance in a DMFC system. The membrane has inherent performance advantages, such as a low methanol crossover (high overall efficiency), while maintaining a high proton conductivity (good electrochemical efficiency) when compared to perfluorinated sulfonic acid membranes such as Nafion. Critically, the membrane provides an extremely low electro-osmotic drag coefficient of approximately one water molecule per proton (versus the 2-3 for Nafion) that minimizes flooding issues at the cathode, which often fatally limit open cathode MEA performance. During this successfully completed DOE program the project team met all of the project goals. The team built and tested over 1,500 MEAs with a wide range of different manufacturing chemistries and process conditions. This project demonstrated that the UNF MEA design could be fabricated with a high degree of reproducibility and repeatability. Some specific achievements include: ??? Durability - The UNF MEA has demonstrated over 11,000 hours continuous operation in a short stack configuration. The root cause of an off-state degradation issue was successfully mitigated by modifying the manufacturing process by changing the wetting agents used in the catalyst printing. The stability of the anode electrode was increased by replacing the anode electrodes with a stabilized PtRu/C catalyst. The overall degradation rate was significantly reduced through optimization of the MEA operating conditions. ??? Performance - The project team optimized the performance of the critical MEA sub-components. By increasing the membrane thickness, the methanol crossover was reduced, thereby increasing the fuel utilization efficiency without sacrificing any electrochemical performance. The reduction in methanol crossover increased the fuel utilization efficiency from 78% to over 90%. The liquid barrier layer was optimized to provide improved reproducibility, thereby improving stack voltage uniformity and reliability. Additionally the barrier layer water permeability was lowered without sacrificing any power density, thereby enabling increased operating temperature. Improvements in the cathode catalyst selection and coating provided an additional 10% to 20% improvement in the MEA performance at the target operating range. ??? Cost - Commercially scalable processes were developed for all of the critical MEA components which led to improved yields and lower overall manufacturing costs. Furthermore, significant steps have been made in improving the process control, which increases MEA uniformity and control of the transport properties to ensure reliable performance. MEAs that were made using these improved process controls showed excellent reproducibility in 40 cell stacks, with as low as ?? 6 mV voltage variation between the MEAs at the target 120 to 150 mA/cm2 operating range. Given that the catalyst is an important driver of the MEA cost, the project team has successfully demonstrated that the catalyst loading can be lowered to 3 mg/cm?? Pt and excellent durability still be achieved with over 2000 hours of operation. UNF and its project partners have met all of the goals and milestones of this project. Additionally, MEA performance has been validated with a 20 W portable DMFC, which was developed in a separate DOE program.